Method of ionitriding objects made of high-alloyed particularly stainless iron and steel

ABSTRACT

Nitrogen hardening of workpieces of stainless and acid resistant chrome and chrome-nickel steels, in a glow discharge in a gas, is enhanced by enriching the gas atmosphere with iron. The enriching is done by atomizing iron from a separate electrode arranged in the glow discharge.

United States Patent inventor Appl. No. Filed Patented Assignee METHOD OF IONITRIDING OBJECTS MADE OF HIGH-ALLOYED, PARTICULARLY STAINLESS, IRON AND STEEL APC Application of Berghaus et al., Ser. No. 233,139, filed 5- 1943 Primary ExaminerF. C. Edmundson Attorney-Bacon & Thomas ABSTRACT: Nitrogen hardening of workpieces of stainless and acid resistant chrome and chrome-nickel steels, in a glow discharge in a gas, is enhanced by enriching the gas atmosphere with iron. The enriching is done by atomizing iron from a separate electrode arranged in the glow discharge.

1 Claim, 2 Drawing Figs. US. Cl 204/177, 204/312, 204/164 Int. Cl ..C23b 11/00, B0 1 k 1/00 Field of Search 204/164, 177; l48/166,165,16

6 5 g I g METHOD 01F HONTTRIIDTNG OBJECTS MADE K llllGlll ALLUYIEID, lPARTIlClUlLARlLY STAlNlLiESfi. HERON AND STEEL The present invention relates to jects made of iron and steel. The term ionitriding" describes a surface treatment of the objects involved in a gas atmosphere containing nitrogen under the action of an electrical glow discharge.

It is well known in the art that objects made of high alloy iron and steel, particularly of stainless and acid-resistant chrome and chrome-nickel steels can only inadequately be hardened by thermal nitriding or bath nitriding. Again, ionitriding in ammonia gas in an electrical glow discharge will produce only an imperfect hardening effect. This is partly due to the fact that oxygen adsorptively held by the surface prevents the nitrogen diffusion into the surface of the object.

Accordingly special methods have been evolved to depassivate the surface in order to eliminate this disadvantage, e.g., by chemical or mechanical means. However, these methods demand considerable labor prior to the actual hardening process and frequently achieve their objective only imperfectly since oxide films may again form after the treatment has been performed.

In extensive tests it has been found that this passive condition of surfaces is only partly due to the poor nitriding quality of high-alloy iron and steel types. The method of ionitriding objects made of high-alloy, particularly stainless, iron and steel according to this invention enables satisfactory nitriding to be obtained also without previous deactivation of the surfaces by chemical or mechanical means. The method according to this invention in which the objects are treated in a gas atmosphere containing nitrogen, particularly ammonia gas, under the action of an electrical glow discharge, comprising a starting cycle limited in terms of time and the nitriding treatment proper is characterized by the fact that hydrogen gas is mixed into the gas-discharging nitrogen during the starting process and that iron particles from an additional electrode arranged in the glow discharge are atomized onto the surface of the objects during nitriding. Together with the atomizing of iron particles, additional alloy elements, by way of example chromium, molybdenum and vanadium, may be atomized if so desired.

This invention further relates to a device for the performance of the method disclosed comprising a metallic container, the supply and discharge means for the treatment gases and an insulated power lead-in, in the interior of which the objects to be treated are arranged in insulated relationship regarding the walls, the said objects being connected to the negative pole of a source of current of which the positive pole is connected to the container. The characteristic feature is a method of ionitriding obvthat, besides the objects forming the cathode of the glow discharge, a ferrous additional electrode is arranged in their vicinity which is insulated relative to the container and connected to a negative pole of the source of current.

A number of embodiments of the invention are described in greater detail with reference to lFlGS. ll through d of the drawings.

FIG. I is a diagrammatic longitudinal section of an embodiment of the invention;

FIG. 2 is a diagram of Vickers hardness il in dependence on depth A.

Ionitriding of objects made of iron and steel is generally known, principally from the BERGHAUS patents, and the same applies to the devices suitable therefor and their operation. Accordingly, a detailed description of the device diagrammatically shown in FIG. ll may be dispensed with. Commonly the said device comprises a metallic container 1 with an insulated power lead-in 2 for connection to the negative pole of a source of current and carrying, in the interior of container 1, the object 3 to be nitrided. Supply and exhaust means 4 are provided for the treatment gases. The metallic housing l is commonly connected to the positive pole of the source of current so that it forms the anode while the object 3 forms the cathode for the glow discharge to be set up in the container.

Arranged in the vicinity of the object 3 is the additional cathode 5 which, according to the embodiment shown in FIG. l, is connected to the workpiece 3 but which may naturally also be connected to a negative pole of the source of current via a particular insulated power lead-in. in the operation of the device, a voltage in the range from 400 to 1,050 volts is maintained between the object 3 and the container l. The glow discharge generated heats the object 3; the treatment temperature should be within 450 and 580 C. The gas pressure in the container 1 should amount to between 0.1 and 10 Tons.

In the present method a so-called starting cycle limited in terms of time is performed as already known from US. Pat. No. 3,018,409 of Applicant. In this case, however, the starting cycle in this case is designed not to obtain a uniform and trouble-free glow discharge for the faultless nitriding of the object 3 as disclosed in the said patent, but to prepare the surface of the object 3 in order to improve the nitriding of objects made of highalloy, stainless and acid-resistant iron and steel types. To this end, hydrogen gas is added to the treatment gas, which may be ammonia gas or some other gas that discharges nitrogen, during the starting cycle; at that time, the gas may preferably largely be hydrogen. Upon termination of the starting cycle, the supply of hydrogen gas is discontinued.

in high-alloy steels such as chrome-nickel steels of which the chromium and nickel content may exceed 50 percent, the iron content of the surface is correspondingly low. This results in that less nitrogen is caused to react at the surface. It has now been found that an increase in the iron supply in the plasma of the glow discharge can result in an increased supply of nitrogen at the surface. Owing to the high kinetic energy of the impinging ions due to the cathode fall of the glow discharge, the object is heated and the nitrided zone is form ed by reactive cathodic atomization. Iron is removed by atomization from the additional cathode 5 and combines with nitrogen in the glow-discharge plasma into iron nitride. This is preferably deposited as a uniform film on the surface of the object 3. The atomized iron nitrides possess a high nitrogen content and decompose on the metal surface, the nitrogen diffusing into the interior of the object and again atomizing iron into the glow-discharge plasma. The iron atomized from the additional cathode may thus be regarded as a decisive nitrogen carrier. As was to be expected according to the diffusion laws, the present method enables a greater hardening depth and more uniform and better ionitriding to be achieved under otherwise similar treatment conditions.

The results obtained in the tests are shown in FIG. 2. The object made of a high-alloy stainless and acid-resistant material (commercially available under the type designation R 35) with 0.03 percent carbon, 20 percent chromium and 25 percent nickel, stabilized with niobium was treated according to the present method with an additional cathode 5 made of common steel; a same object made of the same material was similarly treated without an additional cathode 5. The Vickers hardness HIV in kp/mm. depending on the depth A in mm. for the object treated according to the present method is indicated by curve 6, while curve 7 represents the results of the treatment without an additional cathode. The present method enabled, in comparative tests, a greater hardness at the surface and down to a depth of about 0.l mm. to be achieved. This was clearly visible in photo micrographs a cross section of the objects treated; the greater depth of the hardened surface after treatment of the object according to the present method was readily be recognized in comparing it with a sample of the same object treated according to the conventional method. Expressed in figures, the improvement amounts to:

monia gas, under the action of an electrical glow discharge, in which a starting cycle with a hydrogen enriched gas atmosphere is followed by the nitriding cycle, the improvement for nitriding objects made of high-alloy steel, comprising the by the present method ap- 5 Step of:

enriching the gas atmosphere with iron particles by atomizing the same from the surface of an additional high-iron electrode which is arranged in the glow discharge.

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